docs: Update CONTRIBUTING.md to reference DCO instead of CLA

Replace the outdated CLA (Contributor License Agreement) section with
the current DCO (Developer Certificate of Origin) requirement. The
README.adoc already references DCO, but .github/CONTRIBUTING.md still
pointed to the retired Pivotal CLA.

Also aligned a few other stale references:
- master → main (branch references on lines 6, 29, 40)
- spring-code-of-conduct@pivotal.io → code-of-conduct@spring.io
  (matching spring-cloud-build contributing docs)

Fixes gh-3884

Signed-off-by: Meyon Soo  Kim <39788337+PreAgile@users.noreply.github.com>

Author: PreAgile

.github/CONTRIBUTING.md

@@ -3,21 +3,22 @@
 
 Spring Cloud is released under the non-restrictive Apache 2.0 license,
 and follows a very standard Github development process, using Github
-tracker for issues and merging pull requests into master. If you want
+tracker for issues and merging pull requests into main. If you want
 to contribute even something trivial please do not hesitate, but
 follow the guidelines below.
 
-## Sign the Contributor License Agreement
-Before we accept a non-trivial patch or pull request we will need you to sign the
-[Contributor License Agreement](https://cla.pivotal.io/sign/spring).
-Signing the contributor's agreement does not grant anyone commit rights to the main
-repository, but it does mean that we can accept your contributions, and you will get an
-author credit if we do.  Active contributors might be asked to join the core team, and
-given the ability to merge pull requests.
+## Developer Certificate of Origin (DCO)
+All commits must include a `Signed-off-by` trailer at the end of each commit message
+to indicate that the contributor agrees to the
+[Developer Certificate of Origin](https://developercertificate.org/).
+You can add this automatically by using `git commit -s`.
+
+For additional details, please refer to the blog post
+[Hello DCO, Goodbye CLA: Simplifying Contributions to Spring](https://spring.io/blog/2025/01/06/hello-dco-goodbye-cla-simplifying-contributions-to-spring).
 
 ## Code of Conduct
 This project adheres to the Contributor Covenant [Code of conduct](https://github.com/spring-cloud/spring-cloud-build/blob/main/docs/modules/ROOT/partials/code-of-conduct.adoc). By participating, you  are expected to uphold this code. Please report
-unacceptable behavior to spring-code-of-conduct@pivotal.io.
+unacceptable behavior to code-of-conduct@spring.io.
 
 ## Code Conventions and Housekeeping
 None of these is essential for a pull request, but they will all help.  They can also be
@@ -26,7 +27,7 @@ added after the original pull request but before a merge.
 * Use the Spring Framework code format conventions. If you use Eclipse
   you can import formatter settings using the
   `eclipse-code-formatter.xml` file from the
-  [Spring Cloud Build](https://raw.githubusercontent.com/spring-cloud/spring-cloud-build/master/spring-cloud-dependencies-parent/eclipse-code-formatter.xml) project. If using IntelliJ, you can use the
+  [Spring Cloud Build](https://raw.githubusercontent.com/spring-cloud/spring-cloud-build/main/spring-cloud-dependencies-parent/eclipse-code-formatter.xml) project. If using IntelliJ, you can use the
   [Eclipse Code Formatter Plugin](https://plugins.jetbrains.com/plugin/6546) to import the same file.
 * Make sure all new `.java` files to have a simple Javadoc class comment with at least an
   `@author` tag identifying you, and preferably at least a paragraph on what the class is
@@ -37,7 +38,7 @@ added after the original pull request but before a merge.
   than cosmetic changes).
 * Add some Javadocs and, if you change the namespace, some XSD doc elements.
 * A few unit tests would help a lot as well -- someone has to do it.
-* If no-one else is using your branch, please rebase it against the current master (or
+* If no-one else is using your branch, please rebase it against the current main (or
   other target branch in the main project).
 * When writing a commit message please follow [these conventions](https://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html),
   if you are fixing an existing issue please add `Fixes gh-XXXX` at the end of the commit